1. Field of the Invention
The present invention is generally related to communications systems. In particular, the present invention relates to linearizing Radio Frequency (RF) power amplifiers.
2. Description of the Related Art
Radio Frequency (RF) power amplifiers prepare a signal for transmission by increasing the power of the signal. The signal, such as a television signal, radio signal, or cell phone signal, can be transmitted through the air via an antenna. Other signals, such as those found in cable-TV systems, are transmitted through cables.
One class of RF power amplifiers is known as Class A. Class A amplifiers exhibit good intrinsic linearity but are relatively inefficient. A Class A amplifier can exhibit an efficiency as low as 1%. The efficiency is low because only a small portion of the potential output capability of a Class A amplifier can be used in order to maintain the operation of the amplifier in a linear region. When the overall power output is relatively high, a Class A amplifier can waste a lot of power and in turn, require bulky and expensive thermal management techniques to remove the excess heat generated.
Other classes of RF power amplifiers, such as Class AB, Class B, Class C, etc., are more efficient than a Class A amplifier, but are also intrinsically less linear. Non-linear amplification introduces distortion to the amplified RF signal. The distortion can manifest itself by harmonic frequencies and intermodulation.
Conventional methods attempt to correct the non-linearities of a nonlinear RF power amplifier by introducing Cartesian feedback, feedforward compensation, and predistortion techniques.
Cartesian feedback applies negative feedback to an RF transmitter, which includes an RF amplifier. A sample of the output of the RF amplifier is demodulated and fed back to the input of the transmitter. A disadvantage of Cartesian feedback, as a form of closed loop negative feedback, is that it must be unconditionally stable. Given the delay encountered in modulation and demodulation, the stability requirement gives rise to a relatively narrow operating bandwidth that is impractical for modem wideband code division multiple access (W-CDMA) cellular systems.
Feedforward is a technique in which an additional linear amplifier subtracts the artifacts of nonlinearity from the RF amplifier such that the RF transmitter produces a linearized output. An error signal is derived from comparison of the input to the RF amplifier and the output of the RF amplifier. The input to the RF amplifier is delayed to compensate for the delay through the RF amplifier. The error signal is then amplified by the additional linear amplifier and combined with a delayed output of the RF amplifier to reduce the distortion of the RF amplifier. The output of the RF amplifier is delayed prior to the subtraction in order to compensate for the delay encountered by the error signal through the additional linear amplifier. Feedforward techniques are open loop by nature and can operate over a relatively wide bandwidth. However, the matching of the delays through the RF power amplifier and the additional linear amplifier can be difficult to implement in practice. A mismatch in either or both of the delays seriously undermines the effectiveness of the distortion cancellation.
Predistortion is another conventional technique used to enhance the linearity of a nonlinear amplifier. A digital signal processor (DSP) predistorts the input signal by reference to a predistortion kernel with a complement of the expected distortion of the nonlinear amplifier.
A form of predistortion known as adaptive predistortion further enhances the effectiveness of predistortion by monitoring the output of the RF amplifier and updating the coefficients used by the DSP. A sample of the output of the RF amplifier is demodulated to baseband, and the baseband signal is analog-to-digital converted and applied to the DSP as an input. One disadvantage to present techniques of adaptive predistortion is the relatively limited range and relatively expensive cost of the analog-to-digital converter (ADC). The relatively limited dynamic range of the ADC limits the ability for adaptive predistortion techniques to cancel out nonlinearities.